U.S. Pat. No. 2,920,971 (Stookey) is the basic patent in the field of glass-ceramics. That patent discloses the production of glass-ceramic articles as the controlled crystallization of precursor glass articles by a carefully-defined heat treatment. Thus, a glass-ceramic article is produced by three basic steps: first, a properly formulated glass batch is melted; second, the glass melt is simultaneously cooled to a temperature at least below the glass transformation range and a glass body of desired shape formed; third, the glass body is subjected to a predetermined heat treatment to cause the glass to crystallize in situ.
One of the early commercial products that evolved from the glass-ceramic field was radomes for radar guided missiles. For about 30 years, Corning Glass Works, Corning, N.Y. has manufactured radomes from a glass-ceramic material marketed as Corning Code 9606. That product is highly crystalline with cordierite (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2) constituting the predominant crystal phase. Minor amounts of cristobalite (a polymorph of SiO.sub.2), rutile (TiO.sub.2), and a phase until recently believed to be magnesium dititanate (MgO.2TiO.sub.2) are present also. An approximate analysis of the material, expressed in weight percent on the oxide basis, is reported below:
______________________________________ SiO.sub.2 56.1 Al.sub.2 O.sub.3 19.7 MgO 14.9 As.sub.2 O.sub.3 0.4 TiO.sub.2 8.9 ______________________________________
To be useful as a radome, a material must comply with a complex matrix of mechanical, electrical, thermal, and forming properties. Among these are dielectric constant, loss tangent and modulus of rupture. The dielectric constant affects the velocity of radiation passing through the wall of a radome. The loss tangent defines the quantity of energy absorbed by a material from radiation passing through the material. Thus, high dielectric constants and loss tangents limit the range, and hence utility, of a radome. High mechanical strength is an obvious requirement in view of the vibration and other stresses involved in launching and flight.
In one particular specification, the dielectric constant must not exceed 5.55, the loss tangent maximum is 0.0003 and the modulus of rupture (M.O.R.) should be at least 20,000 psi. The Code 9606 material meets these requirements. It is obvious, of course, that any permissible further treatment of the material must not appreciably degrade these properties.
In order to improve the mechanical strength of a radome, the article, after grinding to a proper prescription, may be subjected to what has been termed a fortification treatment. That treatment comprises subjecting the glass-ceramic to a sequential base-acid leaching process. Thus, the radome is initially contacted with (normally immersed into) an alkaline solution and thereafter, after removing the alkaline solution, it is contacted with (immersed into) an acid solution. That series of steps may be repeated several times in order to achieve the desired effect. As a matter of convenience and economics, a boiling aqueous NaOH solution has constituted the alkaline environment and an aqueous, room temperature H.sub.2 SO.sub.4 solution has provided the acid conditions. The base and acid were customarily removed via rinsing in tap water.
The improvement in strength is deemed to result via healing surface flaws in the body. This phenomenon is due to the cristobalite being leached out of the microstructure (cristobalite is several times more quickly dissolved in hot NaOH solution than is cordierite). The acid acts upon the little residual glass left in the glass-ceramic body. After fortification, Corning Code 9606 demonstrates an enhanced modulus of rupture.
In general, glass-ceramic articles containing cordierite as the predominant crystal phase, but with little or no cristobalite, will demonstrate mechanical strengths, as defined through modulus of rupture measurements, of less than 20,000 psi. In contrast, those cordierite-containing articles with a minor, but significant, amount of cristobalite will evidence modulus of rupture measurements in excess of 30,000 psi after fortification. X-ray diffraction analysis and electron microscopy have indicated that Corning Code 9606 contains about 10% by volume cristobalite.